This invention relates to an electronic all weather landing or navigation system, such as a microwave landing system (MLS), for aircraft and more particularly to means for providing a highly reliable and fail-safe integrity monitoring system which can immediately detect the presence of erroneous guidance data generated by a ground MLS station or processed by an airborne MLS unit.
There is a need for an electronic all weather landing system for aircraft, such as an MLS system, to alleviate the crowded conditions at various airports. One such MLS system which has recently been developed comprises ground based antennas which sweep horizontal and vertical fan shaped beams through the field of interest, nominally the airport approach space, at a known rate through known bounds. Thus, an aircraft in the field of interest will be periodically illuminated by the beams. By means of known techniques and equipment, the illuminated aircraft can determine from the characteristics of the illumination two coordinates of its position in space with respect to the radiating antennas to a very close tolerance. Accurate distance measuring equipment at the ground station and in the aircraft provides the aircraft with the final coordinate so that it can accurately determine its position in space as it approaches the airport and thus can make an expeditious and safe landing even under adverse weather conditions.
Since the landing of an aircraft is generally considered its most vulnerable maneuver, the importance of the aircraft having accurate information as to its position with respect to the airport is readily seen; and it can be understood that erroneous MLS signals during this phase of operation of the aircraft can be extremely dangerous. It is thus quite important that any erroneous guidance data be immediately recognized as such so that corrective action can be taken. The erroneous data can arise by either malfunctioning of the ground station equipment whereby erroneous data is transmitted into the field of interest or by malfunctioning of the airborne equipment whereby the processing of the received data produces erroneous utilization data. Erroneous guidance data can also occur as a result of data propagation phenomena such as multipath or by other electrical interference in the field of interest either intentional such as electronic countermeasures (ECM) or unintentional such as radio frequency interference (RFI). This electrical interference can come from sources within the aircraft such as by malfunctioning of other equipment or improper use of electronic devices by passengers or it can originate from other aircraft or from the ground.